The presence of significant quantities of H.sub.2 S and CO.sub.2 in various "sour" industrial gaseous streams poses a persistent problem. Although various procedures have been developed to remove and recover these contaminants, most such processes are unattractive for a variety of reasons. Accordingly, there remains a need for an efficient and low cost process to remove these undesired materials.
While some work has been done in the area of removal and conversion of H.sub.2 S to sulfur by use of iron complexes in waste gas streams, such processes generally ar inapplicable directly to purification of hydrocarbon feedstock, coal gasification, or natural gas streams having high ratios of CO.sub.2 to H.sub.2 S. For example, U.S. Pat. No. 3,266,320 (Meuly, et al.) discloses the removal of H.sub.2 S from fluids passing through or leaving industrial processes by adding to the polluted streams certain chelates of polyvalent metals. The chelates are added in an amount which is stoichiometrically substantially less than the amount of the pollutant, and the chelate- and pollutant-containing stream is contacted with oxygen. The preferred chelates are the iron, cobalt, and nickel chelates of acetyl acetone, cyclopentadiene, ethylene diamine tetraacetic acid, N-hydroxyethyl ethylene diamine triacetic acid, gluconic acid, tartaric acid and citric acid. The chelate is said to catalyze the reaction.
As indicated, however, this procedure is unsuited to gaseous hydrocarbon feedstock, coal gasification stream, or natural gas treatment since no provision is made for CO.sub.2 removal. More importantly, the use of oxygen with the materials contemplated would be intolerable. Finally, the amounts of chelate employed in the patent are merely catalytic, and do not give the desired degree of H.sub.2 S removal from gaseous streams.
Similarly, U.S. Pat. No. 3,622,273 (Roberts and Johnson) discloses the removal of H.sub.2 S from a gaseous stream by contacting the gaseous stream with a solution containing, by weight, from 0.005 to 20 percent of a ferric ion complex, from 25.0 to 99.945 percent of water, and from 0.05 to 10.0 percent of a buffering agent selected from the group consisting of alkali metal carbonate and alkali metal bicarbonate. According to the patentees, known complexing agents, such as nitriloacetic acid and ethylene diamine tetraacetic acid, present stability problems when employed in H.sub.2 S removal. To overcome these problems, the patentees specify the addition of a buffer to the treating solution, in the amount and type indicated previously. The ferrous complex resulting from reaction of the ferric complex with the H.sub.2 S may be regenerated in a separate zone and reused indefinitely.
Nonetheless, there remains a need for an economical and efficient method of H.sub.2 S and CO.sub.2 removal which avoids the stability problems mentioned and the requirement of added buffering, etc. The present invention satisfies that need.